Liquid crystal display panels and systems are disclosed, for example, in such U.S Patents as U.S. Pat. No. 3,862,360 issued to Dill et al on Jan. 21, 1975; U.S. Pat. Nos. 3,863,332 issued to Leupp et al on Feb. 4, 1975; and 3,861,783 issued to Dill et al on Jan. 21, 1975.
In the first mentioned Dill patent, it is explained in lines 5-10 of Col. 7 that the reflective electrode used therein which contacts the liquid crystal material may be made of aluminum, nickel, chromium, gold, or silver deposited by standard evaporation or sputtering techniques. This teaching represents the standard practice of the prior art and is satisfactory for reflecting broad band or black and white images.
However, for many display applications such as watch readouts, instrument panels and the like, it is desirable to provide a colored display pattern. By means of the use of the dielectric reflector process disclosed herein, it is possible to achieve not only higher reflectivity for the reflecting electrode than is possible with the metals discussed above, but also to achieve certain desirable colors such as silver, yellowish gold, magenta red, blue and green. The particular color selected depends upon controlling the thickness of the silicon dioxide and silicon layers. The percent of reflectivity can also be further increased by the use of added sandwiched layers. The resulting display is suitable not only for use in t.v. displays but also in watches, bargraphs, etc.
While the optical properties of layers of semiconductor materials have been utilized in various other optical devices, the prior art has not recognized their value as a wavelength selective reflect electrode. In U.S. Pat. No. 3,726,585 issued to Fedotowsky on Apr. 10, 1973, there is shown, for example, a multilayer arrangement of semiconductive layers which is arranged to function as a modulator to vary the intensity of a monochromatic light beam which is passed through the multilayer arrangement near the critical angle of incidence. One of the layers acts as an optical resonator and the refractive index of the other layer is varied by application of an electrical voltage. This patent relates primarily to a general purpose electrically tunable filter rather than to the use of a particularized reflector.
Of course, anti-reflection coatings of multiple layers have been known in the prior art for some years. Such a coating is illustrated, for example, in U.S. Pat. Nos. 3,235,397 and 3,356,522.
In U.S. Pat. No. 3,679,291, there is shown a filter formed by a multilayer coating which is neutral in transmission and has asymmetric reflectance. Again, however, the filter is formed and used in a manner quite different from the reflector taught herein.
Finally, U.S. Pat. No. 3,910,033, discloses a watch dial for an analog readout watch which is covered with a transparent film of material which may be silicon dioxide, magnesium fluroide, or the like but which is preferably sapphire in order to give rise to interference between the light reflected from the front and rear surfaces of the film thereby producing interference colors. In the environment of a liquid crystal display panel, such a single film coating is not sufficient to produce either the increased reflectivity or color selectivity desired and achieved by the present invention.
It is therefore an object of this invention to provide a multilayer arrangement of thin films of dielectric material suitable for use as a reflector with liquid crystal display panels.